1. Field of the Invention
The present invention relates to a WDM optical transmission system and a WDM optical transmission method for transmitting a wavelength division multiplexing (WDM) signal light including a plurality of different wavelengths. More specifically, the present invention relates to a technique for reducing deterioration of transmission characteristics due to interaction between an intensity-modulated optical signal and a phase-modulated optical signal.
2. Related Art
There is a demand for an increase in transmission capacity of, for example, submarine optical cable systems, accompany the increase in communication traffic. To meet such demands, it can be generally considered to lay a new submarine optical cable and construct a submarine terminal. There is another method of newly adding a submarine terminal to an optical cable (dark fiber), which has been laid already but not used. An upgrade method is also known, in which the transmission capacity is increased at a low cost by newly adding an optical channel to an already introduced optical communication device.
The above upgrade method includes a method of increasing the optical channels by connecting an optical transmitter and an optical receiver to vacant ports of a multiplexer and a branching filter in an existing WDM optical transmission device. Moreover, there is another method in which optical branches are respectively provided on a transmission side and a reception side of the terminals corresponding to existing optical signals (optical channels) CH1 to CH4, as shown in FIG. 11, relative to an initial configuration of the WDM optical transmission system such as shown in FIG. 10, to increase terminals corresponding to the new optical channels CH5 to CH8. FIG. 12 shows an arrangement example in a wavelength range of the existing optical channels CH1 to CH4 and the new optical channels CH5 to CH8 in such an upgrade method. In this example, optical channels CH5 and CH6 to be increased are arranged on a short wavelength side of the existing optical channels CH1 to CH4, and optical channels CH7 and CH8 to be increased are arranged on a long wavelength side of the existing optical channels CH1 to CH4.
Incidentally, in a system for overland communications, Non Return to Zero (NRZ) format is generally used as the optical signal modulation method, whereas in a system for submarine communications, Return to Zero (RZ) format is generally used as the optical modulation method. In the RZ format, the optical transmitter has a complicated configuration, but has an advantage in that reception sensitivity is excellent and signal deterioration is relatively small even when the optical fiber is used for long-distance transmission.
A phenomenon causing the signal deterioration in the WDM optical transmission system using the RZ format optical signal is attributable to a non-linear effect of the optical fiber, and specifically, the phenomenon includes self phase modulation (SPM) and cross phase modulation (XPM). The SPM is a phenomenon in which a refractive index of the optical fiber changes corresponding to the power of the optical signal propagating on the optical fiber, thereby applying phase modulation to the optical signal. Since the spectrum of the optical signal expands due to the phase modulation, the waveform of the optical signal is distorted corresponding to a dispersion characteristic of the optical fiber. Moreover, the XPM is a phenomenon in which the refractive index of the fiber changes due to the optical power of the optical channels having adjacent wavelengths, thereby applying phase modulation to the optical signal, and the wavelength of the optical signal is distorted corresponding to the dispersion characteristic of the optical transmission path.
As a technique for suppressing this deterioration of the WDM signal light due to the XPM, there is a technique for suppressing generation of the XPM by arranging the wavelength of the optical transmitter so that bit patterns of optical signals having wavelengths adjacent to each other cause walk-off of at least a ½ bit period relative to each other in an effective length of the optical fiber (for example, refer to Japanese Unexamined Patent Publication No. 7-66779). Moreover a technique in which different chromatic dispersion is imparted to optical signals having wavelengths adjacent to each other at the time of transmission, and the chromatic dispersion thereof and chromatic dispersion of the optical transmission path are compensated on the reception side, to thereby suppress crosstalk between adjacent optical signals is also known (for example, refer to Japanese Unexamined Patent Publication No. 2000-183815).
As the non-linear effect of the optical fiber, there is also four wave mixing (FWM), other than the above described SPM and XPM. The occurrence of FWM can be avoided by giving an appropriate difference in the propagation speed between respective optical channels of the WDM signal light. Specifically, by using, for example, an optical fiber having chromatic dispersion of about −2 ps/nm/km, the occurrence of FWM can be avoided.
Moreover, in relation to the modulation method of the optical signal, recently, to improve the reception sensitivity further, application of a differential phase shift keying (RZ-DPSK) format in which information is carried on an optical phase is being studied (for example, refer to J. -X. Cai et al., “RZ-DPSK Field Trial Over 13100 km of Installed Non-Slope-Matched Submarine Fibers”, Journal of Lightwave Technology, Vol. 23, No. 1, January 2005, pp 95-103). In this RZ-DPSK format, the configuration of the optical transmitter/receiver becomes more complicated as compared to the RZ format, however, it is expected that the reception sensitivity can be improved by about 3 dB as compared to the RZ format.
In the above described upgrade method, when it is considered to apply the RZ-DPSK format to an increased optical channel, since the RZ-DPSK optical signal has smaller spectral line broadening, an advantage is expected where optical signals can be arranged in higher density than in the RZ format. Moreover, even when cumulative dispersion in the transmission path is large and, in the RZ format, desired transmission quality cannot be ensured due to a large penalty, RZ-DPSK optical signals can be increased, because of the excellent reception sensitivity in the RZ-DPSK format.
However, in the above described conventional upgrade method, when application of the phase modulation method such as RZ-DPSK to the optical signal to be increased is assumed, if the modulation method of the existing optical signal to be adjacent to the optical signal to be increased in the wavelength range is not phase modulation but intensity modulation such as RZ, the transmission characteristic of the phase-modulated optical signal to be increased may deteriorate due to the interaction with the intensity-modulated optical signal. In other words, the phase-modulated optical signal carries information on the optical phase, but there is a possibility that information of the intensity-modulated optical signal may be carried on the phase of the phase-modulated optical signal as a noise due to the cross phase modulation (XPM) from the adjacent intensity-modulated optical signal. If such a phenomenon occurs, the transmission characteristic of the phase-modulated optical signal deteriorates, which causes a problem.
The above problem will be specifically described with reference to FIGS. 13 to 15. First as shown in FIG. 13, a phase variation (thick line) caused in the adjacent channel by the RZ optical signal is different between data “1” corresponding to a high level of the optical waveform (thin line) and data “0” corresponding to a low level thereof. On the other hand, as shown in FIG. 14, the phase variation caused in the adjacent channel by the RZ-DPSK optical signal is constant irrespective of data “1” and “0”. Therefore, as shown in FIG. 15, the RZ-DPSK optical signal causes signal deterioration due to disturbance, that is, phase modulation due to an influence of a phase change affected by the adjacent RZ optical signal. Such deterioration of the RZ-DPSK optical signal is hardly suppressed even if the conventional technique such as optimization of wavelength arrangement is applied, taking into consideration the aforementioned bit pattern of the adjacent optical signal.